The present disclosure is directed generally to transmitters for data in a network and, more specifically, to an improved transmission scheme for wireless communication, for example, in a large area network (LAN).
Although the present invention will be described with respect to wireless communication and, specifically, the IEEE Standard 802.11, the principles also apply to other transmitting schemes which involve data modulation, spreading and over-sampling ETC.
IEEE Standard 802.11 (Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications) is a protocol standard for wireless LAN communication. Its physical layer in the transmitter is responsible for encoding and modulating a packet into baseband signals. As illustrated in FIG. 1, for given a sequence of bits of a packet, the physical layer:
1. modulate the bits or symbols (one bit per symbol for DBPSK and 2 bits per symbol for DQPSK) with DBPSK (for 1 mbps) or DQPSK (for 2 mbps) or other modulation systems to get a sequence of (I, Q) pairs at 10;
2. spread I and Q with Barker sequence to get eleven chips for each I and Q bit at 12;
3. over-sample each chip by a factor of M, say M=4, to get a sequence of I samples and Q samples at 14;
4. feed the I and Q samples to a pulse-shape filter 16;
5. forward the filtered I and Q samples to a RF modulator 18; and
6. transmit the RF modulated I and Q samples.
Although each of the steps are necessary, they are time consuming. Combining or implementing any or more of the steps in a more efficient way will increase the transmission speed of the transmitter.
The present method performs steps 1, 2, 3 and 4 using one or more look-up tables, therefore substantially decreasing the processing time. The present invention is a method of differentially modulating, spreading, over-sampling and filtering a stream of bits forming symbols for transmission including determining an Ii, and a Qi for each symbol Si using differential modulation with respect to a previous symbol Si−1 having an Ii−1 and a Qi−1. A row of a matrix for I is indexed using two of Ii−1/Qi−1, Ii/Qi, and Si and providing the samples in the indexed row as a spread, over-sampled and filtered output of Ii. Also, a row of a matrix for Q is indexed using two of Ii−1/Qi−1, i,/Qi, and Si and providing the samples in the indexed row as a spread, over-sampled and filtered output of Qi. These values for Ii and Qi are then RF modulated and transmitted.
The determining of I and Q includes using one or more of the following differential phase shift keying (DPSK): binary phase shift keying (BPSK) and quadrature phase shift keying (QPSK). The method of determining the Ii, Qi, uses a look-up table indexed by the previous Ii−1 Qi−1 and the present symbol Si. The method of determining Ii Qi is performed in one of in series with, in parallel with and simultaneously with indexing the I and Q matrices.
The matrix has n rows equal to the number of possible combinations of values of two of the previous Ii−1, Qi−1, the present Ii, Qi, and the present symbol Si and m columns equal to a spread factor times an over-sample factor. Prior to performing the method, the spreading, over-sampling and filtering are determined and stored in the rows of the matrix.
The present invention is also directed to a method of transmitting a stream of data bits which form a symbol and includes determining for each symbol an I and a Q. A matrix is indexed using the I and Q to provide a spread, over-sampled and filtered output of the I and Q. This output is RF modulated and transmitted. A matrix for I and a matrix for Q are indexed using two of the present symbol Si, the present Ii, Qi and the previous Ii−1, Qi−1. The I and Q are determined using differential modulation of a present symbol Si with respect to the previous symbol Si−1.
The present invention is also directed to the matrix itself having n rows equal to the possible combinations of values of Ii, Qi, and m columns equal to the spread factor times an over-sample factor. Each row includes values corresponding to the spread, over-sampled and filtered samples I, Q. The modulated I, Q's are differentially modulated for a present Ii, Qi with respect to a previous Ii−1, Qi−1, and the n rows equal to the possible combinations of values of two of the previous Ii−1, Qi−1, the present Ii, Qi, and the present symbol Si.
These and other aspects of the present invention will become apparent from the following detailed description of the invention, when considered in conjunction with accompanying drawings.